THE SKIN. 429 



In Nemerteans — 



(1) The lateral nerve -cords sometimes approach one another 



ventrally, and in rare cases dorsally. An approximation 

 dorsalwards, and union on that surface, would result in a 

 double dorsal nerve-cord. 



(2) The firm dorsal sheath of the proboscis may correspond to a 



notochord. 



(3) The proboscis itself may correspond to the hypophysis or 



pituitary process characteristic of Vertebrate brains. 



(4) Two ciliated slits on the head may correspond to a pair of gill- 



clefts. 



(5) There is no segmentation, but the branches given off from the 



nerve-cords are sometimes serially arranged. 

 It must be noted that those who support these theories do not assert 

 that any Nemertean or Annelid is in the direct line of Vertebrate 

 ascent. They simply emphasise the demonstrable affinities. When 

 these are thoroughly worked out, it will be possible to say which 

 Invertebrate types are most nearly related to Vertebrates. 



Structure and Development of Vertebrates. 



Having separately discussed the Hemichorda, Urochorda, 

 and Cephalochorda, we propose in this chapter to discuss 

 the general structure of Craniata and the development of 

 some of the important organs. 



Skin. — This forms a continuous covering over the surface 

 of the body, serves as a protection to the underlying tissues, 

 in some instances retains its primitive respiratory sig- 

 nificance, and is frequently concerned in the excretion of 

 waste and the regulation of the body temperature. As one 

 or other of its many functions predominates, there are cor- 

 responding structural modifications. One function which 

 we find oftenest emphasised at the expense of the others, 

 is that of protection, and yet the extinct Glyptodon, the 

 sluggish Chelonia, the decadent Ganoids, seem to indicate 

 that this, in itself, or in its correlated variations, is not 

 conducive to the continuance of the species. 



The skin includes — 



(a) The epidermis, usually in several layers, the outer "horny" 



stratum corneum, the inner " mucous " stratum Malpighii, 



or mucosum ; both derived from the ectoderm or epiblast 



of the embryo. 



(d) The dermis, cutis, corium, or under-skin, derived from the 



mesoderm or mesoblast of the embryo. 



From the epidermis are derived feathers, hairs, and some kinds 01 



scales. The dermis, as is natural when we consider its origin fiom the 



